(i) Field of the Invention
The present invention relates to a method for purifying glycine. More specifically, the present invention relates to a method for obtaining substantially colorless glycine from an aqueous glycine solution. Glycine is a useful compound which is widely used as a raw material for food additives of processed foodstuffs, agricultural chemicals and medicines.
(ii) Description of the Prior Art
Heretofore, as methods for preparing glycine, there are known mainly (1) an amination method using monochloroacetic acid, (2) the Strecker method, and (3) the hydantoin method.
(1) In the amination method using monochloroacetic acid, the raw materials are expensive, and an ammonium salt of a mineral acid such as ammonium chloride, iminodiacetic acid and nitrilotriacetic acid are produced, and a complex process is required to separate these products from glycine.
(2) The Strecker method comprises reacting ammonia and glycolonitrile which can be synthesized from hydrocyanic acid and formalin, and then carrying out hydrolysis with an alkali such as sodium hydroxide to form a metal salt of glycine. Thus, it is impossible to directly produce free glycine.
As a technique for preparing free glycine from this metal salt of glycine, there are known (a) a process for ion exchanging the metal salt of glycine with an acidic cation exchange resin and (b) a process for neutralizing the metal salt with a mineral acid such as sulfuric acid to effect desalting. In the case that an ion exchange resin 1 is used, a very large amount of the resin is necessary, and on the other hand, in the case that sulfuric acid is used as a mineral acid, it is difficult to separate, from glycine, sodium sulfate which is a neutral salt and iminodiacetic acid which is one of the by-products, and the treatment of these neutral salts is troublesome.
(3) The hydantoin method comprises reacting glycolonitrile, which can be synthesized from hydrocyanic acid and formaldehyde, with ammonia and a carbon dioxide gas in the presence of water to form hydantoin, and then hydrolyzing the same to prepare glycine. If hydrocyanic acid is replaced with sodium prussiate or potassium prussiate as one of the raw materials, or if an alkali such as sodium hydroxide is used in the hydrolysis, free glycine cannot be directly prepared as in the Strecker method. As a result, the hydantoin method involves problems of the treatment of the neutral salts and the like.
In this hydantoin method, a method for directly obtaining glycine without using an alkali such as sodium hydroxide (hereinafter referred to as "direct hydantoin method") is considered to be an economical process for preparing glycine without any environmental pollution, since by-products such as sodium sulfate and the like are not formed.
As such a direct hydantoin method, there is, for example, a method in which hydrogen cyanide, aldehyde, ammonia and carbon dioxide are heated at 100.degree. C. or more in an aqueous solvent (U.S. Pat. No. 3,536,726).
As a decoloring technique for crude glycine obtained by the above-mentioned method, a method is known in which an aqueous crude glycine solution is usually treated with an ion exchange resin or active carbon. In addition, crystallization from methanol or washing with methanol is also effective for the decoloring.
In order to obtain free glycine by the Strecker method, a metal salt of glycine is neutralized usually with sulfuric acid or the like so as to be pH 6 to 7 or so which is convenient to crystallize glycine, followed by desalting, thereby obtaining glycine in the state of an aqueous glycine solution containing neutral salts or an aqueous glycine solution from which most of the neutral salts are separated. Afterward, decoloring is made with active carbon, an anion exchange resin or the like.
Japanese Patent Publication No. 54-1686 discloses a purification process for glycine in which a crude aqueous glycine solution having a pH of 7 or less is obtained by the Strecker method, the monochloroacetic acid method or the like is treated at 50.degree. C. or less with a weakly basic anion exchange resin or a moderately basic anion exchange resin.
If active carbon is used for decoloring, the crude aqueous glycine solution is temporarily decolored, but when heated for concentration, it is colored again. Therefore, the prior art describes that when active carbon is used, it is necessary to repeat recrystallization for high purity and a large amount of the active carbon is required.
On the other hand, with regard to the decoloring technique of colored glycine obtained by the direct hydantoin method, U.S. Pat. No. 3,536,726 discloses that a reaction solution is directly treated with active carbon and then crystallized from methanol to prepare glycine, and Japanese Patent Laid-open No. 49-127915 discloses that a reaction solution is concentrated under reduced pressure to precipitate crystals of glycine, and the latter are then decolored with active carbon and recrystallized from water-methanol to prepare white glycine.